1. Field of the Invention
This invention relates to the production of hydrocarbons from a subsurface reservoir. More specifically, the invention pertains to a composition and method for reducing permeability of selected portions of a reservoir to prevent the bypass of fluids.
2. Background of the Invention
This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present invention. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Hydrocarbons are generally produced from rock matrices existing many hundreds or even thousands of feet below the earth surface. To recover such hydrocarbons, one or more wells are drilled to the depth of the hydrocarbon-bearing reservoir. Once the wells are completed, reservoir fluids are drained from the rock matrix and into a wellbore where they are produced to the earth surface.
The reservoir from which hydrocarbons are produced has various characteristics. These include temperature, pressure, porosity, and permeability. Ideally, permeability within the reservoir is uniform. The presence of areas with permeability higher than that of other portions of the hydrocarbon-bearing rock can cause an uneven flow of fluids through the rock matrix and into the wellbore.
Subsurface zones with permeability higher than the average of the producing interval are sometimes referred to as “thief zones.” A thief zone may be a naturally-occurring region of the reservoir. Such regions can exist as layers formed during the natural geologic depositional process. Streaks of high porosity and permeability are common in carbonate reservoirs. These regions can also occur in sandstone reservoirs in the form of fractures or other channels of very high porosity and permeability.
In another instance, a thief zone may be created during the primary production of hydrocarbons, forming a channel or “wormhole.” Such channels may be caused by the production of sand from an unconsolidated formation along with the oil or water. These channels can hasten production of high volumes of water during primary recovery operations when the channel connects the producer to a source of water, such as an aquifer.
Thief zones can also significantly alter the sweep of injected fluids in a secondary or tertiary flood operation such as water flooding or gas flooding. Such thief zones can cause bypassing of oil and early breakthrough of fluids, such as water or gas, injected into the reservoir from injection wells to displace oil, resulting in large amounts of the displacing fluid being produced from production wells. Thief zones, therefore, can cause inefficient production of the desired hydrocarbons and reduce oil recovery.
Several fluid diversion and permeability profile modification techniques for reducing the permeability of thief zones have been disclosed in the prior art. Many of these have attempted to modify the vertical permeability profile of the reservoir at the well face or within a few feet of the wellbore. However, thief zones can extend to great distances from an injection or production well. Thus, measures taken to plug thief zones within a few feet of a wellbore are of limited value.
In some cases, the depth of treatment is restricted because the technique employs injection of fluids that only penetrate a small distance into the reservoir. In other instances, depth of treatment may be limited because the fluid that is injected relies upon a chemical reaction to induce fluid thickening or relies upon phase change that occurs rapidly and limits the injection time. Such treatment methods are often ineffective because adjacent strata within the subsurface production zone, having both high and low permeability, are often in fluid communication throughout a substantial portion of the reservoir between wells. Therefore, even if the high permeability zone is plugged near-wellbore, cross-flow from lower permeability pay to the thief zone at greater distances from the wells reduces the effectiveness. In this instance, fluids injected to displace oil can still bypass much of the hydrocarbon-bearing rock.
Some processes have been directed at modifying the permeability profile of the formation during drilling. However, these processes are primarily intended to minimize the loss of drilling mud. Such processes also fall into the class of near-wellbore treatments.
Some of the above technologies are disclosed in the patent literature. Patents which relate to the sealing of a subsurface zone include U.S. Pat. Nos. 3,509,951, 3,965,986, 4,192,753, 4,475,594, 4,732,213, 5,836,390, 6,059,036, 6,302,209, 6,632,778, and 6,716,282.
Wax treatments have been previously attempted in the field to plug thief zones. However, these wax treatments have used only a single-phase wax heated above its melting range as the injected fluid. A disadvantage of using a single-phase wax is that its viscosity is very low (approximately 1 centipoise (cp)). The low viscosity permits the injected melted wax to finger through resident fluids, such as water. They are, therefore, not effective in displacing the fluid residing in the thief zone to be plugged, leaving regions of the thief zone without solid wax.
Hence, a need exists for a plugging agent that can be injected into a subsurface formation to substantially fill a thief zone. A need further exists for a plugging agent that can be injected into a high permeability zone that extends a considerable distance from the wellbore. A need further exists for a plugging agent that, after setting in a high permeability zone, provides resistance to flow at pressure gradients experienced within the reservoir during normal operations. Further, a need exists for a plugging agent that is temperature sensitive and can be reversed after setting. Thus, the plugging agent is amenable to being cleaned up or removed from lower permeability zones inadvertently invaded near-wellbore during placement of the solution.